LEDs are optoelectronic devices, which emit light by recombining injected electrons and holes radiatively. Depending on the bandgap of the active material in a particular optoelectronic device, LEDs can emit at a wide range of wavelengths from ultraviolet to infrared. However, the wavelengths of light which are of major interest are in the visible region. LEDs emitting in the visible spectrum (typically from ˜400 nm (purple) to ˜700 nm (red)) are visible to one's eye and are thus useful for illumination purposes. In order to emit light at visible wavelengths, the group III and V elements (i.e., elements in the third and fifth columns of the Periodic Table, respectively) which are often used are gallium (Ga), indium (In) and nitrogen (N). Such materials are often doped with impurities from other columns of the periodic table to allow electrical activity, which in turn generates light via the recombination of an electron from a conducting state to a valence state.
The devices above are referred to as being of the (In,Ga) N material group. LEDs fabricated from this material system have been utilized. LEDs typically include monochromatic light sources which emit with single spectral peak and a narrow linewidth (e.g., ˜30 nm). LEDs fabricated using the (In,Ga) N material system can be made to emit monochromatic light ranging from ˜380 nm (near UV) to ˜540 nm (i.e., green) by changing the indium composition in the material system. LEDs, with their monochromatic nature, are useful in applications such as light indicators, where only a single color is required.
White light, on the other hand, is broadband, polychromatic light which cannot be generated directly with a single LED. However, if an LED can be made to generate light at a number of discrete or continuous wavelengths, the resultant spectrum may be polychromatic and the emission from such an LED will appear as white. This may be useful because white light is often ideal for illumination purposes. LEDs as illumination light sources may be superior to previous lighting technologies such as incandescent lamp and fluorescent tubes, in terms of luminous efficiency, lifetime and spectrum pureness.
There are two major conventional methods of making broadband LED light sources. The first method makes use of phosphors for color down conversion. Phosphorescent materials that emit light when exposed to certain wavelengths of radiation are traditionally used for color conversion in light-emitting diodes (LEDs). A device may emit a high-energy photon, and the phosphor may absorb it and then re-emit a lower-energy and thus differently colored photon.
Such phosphors absorb shorter-wavelength photons and re-emit longer wavelength photons. For white light emission, green and red light-emitting phosphors may be used. It should be observed that any form of color conversion involves energy losses. While green phosphors can have quantum efficiencies of up to 90%, quantum efficiencies of red phosphors are typically limited to around 40%. This, in turn, translates to low wall-plug efficiency.
In such color down conversion schemes, a shorter wavelength monochromatic LED, such as an InGaN LED emitting at 460 nm (blue), may be used as an excitation light source. Such light may be used to excite luminescence in phosphors emitting at longer wavelengths, such as green and red. A resultant light consists of components from different parts of the visible spectrum, and is thus considered broadband light. Since the phosphor particles are small (e.g., on a nanometer scale) and indistinguishable to the naked eye, the emitted light appears as white if the proportions of the different colors are right. This form of white light generation is similar to that employed in fluorescent tubes.
However, there are many drawbacks associated with phosphors, including limited lifetime, Stokes-wave energy loss, low reliability and low luminous efficiency.
Another method of making a broadband LED light source is to mount discrete LED chips onto a single package. These are often called multi-chip LEDs, where LEDs emitting at the primary colors of light (i.e., blue, green and red) are mounted onto a single package. However, white light emission cannot be achieved using this technique. Each LED chip is typically over 100 microns in dimension, while the separation of LED chips is of the same order. As a result, the colors are not homogenized and therefore appear as discrete colors to the naked eye unless placed at very far distances, by which time an LED's intensity has dropped immensely.